In recent years, for example, a technology, such as a wireless ad-hoc network, for performing End-To-End communication between mobile terminals without the intervention of a wireless access point has been known. Realization systems which are presently and widely known on packet relay control for performing End-To-End communication between mobile terminals without the intervention of a wireless access point are broadly divided into a “table drive type (see, for example, Non Patent Literatures 1 and 2 below)” and a “storage transfer type (see, for example, Non Patent Literatures 3 and 4 below)”.
In the storage transfer-type packet relay system described above, a relay mobile terminal storing data is in a state where it can simultaneously and wirelessly communicate with two or more adjacent mobile terminals, and when the relay mobile terminal has the two or more mobile terminals as adjacent nodes, in order to enhance a probability that the data reaches a final destination, a routing control system described below is adopted. Specifically, a data relay operation in which each relay mobile terminal storing data transfers a plurality of copies of data to a plurality of adjacent nodes is sequentially repeated, and thus a large number of copies of data are spread between mobile terminals which can communicate with each other. The mechanism of such data spread is known as infection-type routing control (Epidemic Routing), estimation-type routing control (Estimation Routing), coding-type routing control (Coding Routing) or the like (see for example, Non Patent Literatures 9 to 11 below).
A “star connection-type packet relay system” disclosed in Non Patent Literatures 5 to 8 is an improved-type system in which in the storage transfer-type packet relay system described above, communication between mobile terminals is controlled in units of a terminal group. In the star connection-type packet relay system, in each terminal group, around a group owner (GO: Group Owner) serving as a parent node terminal, one or more mobile terminals serving as child node terminals perform wireless connection so as to form a star-type network connection topology.
As described above, in the wireless network environment for performing communication between mobile terminals without the intervention of the wireless access point, it is expected that in the future, a wide variety of packet relay systems are used to operate the network. The wide variety of packet relay systems described above include the table drive type, the storage transfer type and the like, and in the storage transfer-type system, as a routing control system for relaying and transferring packets, various systems such as the infection-type routing control (Epidemic Routing), the estimation-type routing control (Estimation Routing) and the coding-type routing control (Coding Routing) are used. Consequently, it is also expected that in the future, in the wireless network environment described above, in order to seamlessly realize packet relay control on transfer data between mobile terminals, the network is operated in the form in which mutual cooperative operations are performed between different packet relay control systems. It will be necessary that each of the mobile terminals which relay and transfer data be continue a packet relay operation while switching to an appropriate packet relay control system according to the change in conditions (for example, the stability of a wireless channel state, terminal mobility and a terminal density within an adjacent area) under which the terminal is placed.
Even if in the wireless network environment described above, the various packet relay systems described above are appropriately used according to the conditions or are used by being combined, the following problems are still present. In other words, a problem in the reachability of communication, a problem in the scalability of a communication capacity, a problem in the efficiency of communication and a problem in the optimization of the efficiency of the consumption of power in a terminal caused by the systems are not solved. Specifically, the problems are as follows.
(1) In the table drive-type relay system, when the mobility of each mobile terminal is increased, and a wireless channel state between mobile terminals becomes unstable, the topology of the entire network becomes unstable, with the result that a packet arrival failure rate in the End-To-End is rapidly increased. In other words, in the table drive-type relay transfer system, the End-To-End communication route becomes very unstable.
(2) In the table drive-type relay system, routing information is constantly exchanged between a plurality of mobile terminals, and thus terminal batteries are remarkably consumed or the communication capacity is reduced.
(3) In the storage transfer-type relay system, in the data transfer of the End-To-End, a communication delay occurs randomly and without limitation. Moreover, in the storage transfer-type packet relay transfer system in which the End-To-End communication route is not determined until the packet reaches the final destination, it is impossible to guarantee the arrival of the packet in the End-To-End regardless of whether or not the communication state is satisfactory.
(4) When the amount of communication relayed and transferred exceeds the upper limit of the communication capacity which can be supported according to the buffer-memory capacity on each relay terminal, since transfer data overflows from the buffer-memory, in the storage transfer-type packet relay transfer system, a drop in the relay transfer data can be produced.
(5) As a routing control system adopted by the storage transfer-type system, the system illustrated above such as the infection-type routing control (Epidemic Routing), the estimation-type routing control (Estimation Routing) or the coding-type routing control (Coding Routing) has the following problems. Specifically, in these routing control systems, the probability that data reaches the final destination can be increased whereas a very large number of data transfers are produced simultaneously and in parallel between a large number of relay mobile terminals within the network, and thus the efficiency of the communication and the efficiency of the terminal power consumption are lowered.
As the point that is common to the problems (1) to (5) described above, as the number of hops in the End-To-End communication route from the transmission source terminal to the final destination terminal within the wireless network is increased, the following problems are more likely to occur. Specifically, disadvantageously, when a table drive-type relay system is used, as long as a wireless channel state within a wireless network and a radio wave condition are not maintained to be ideal, the period during which the communication route is stably present is shortened, and the probability that the communication route is stably maintained is lowered. Moreover, disadvantageously, when a storage transfer-type relay system is used, as the number of hops in the End-To-End communication route is increased, the probability that the packet reaches the final destination is lowered, with the result that a communication delay time necessary for the arrival is increased.
On the other hand, in the “star connection-type relay system” described above, due to the following reasons, most of various problems on the table drive type and the storage transfer type which are examined as described above are overcome.
In the “star connection-type relay system”, the topology of the entire network is simplified into a very simple star-type topology. Hence, in this system, not only relay transfer control on the packet performed on the mobile terminal is simplified but also the processing burden of packet relay transfer control by a CPU on the mobile terminal is reduced. Furthermore, in this system, it is possible to significantly reduce the number of times a message is transmitted and received which is necessary for sharing data between mobile terminals within a terminal group as compared with the “table drive type” and the “storage transfer type”. Consequently, when the mobile terminals within the terminal group share data, the terminal power consumed by each of the mobile terminals within the terminal group can be saved. The advantage described above is not limited to the “star connection-type relay system” but is common to a relay system in which a large number of mobile terminals are grouped into terminal groups and in which packet relay transfer is performed by communication between the terminals within each of the terminal groups. In the following description, the relay system in which, as described above, a large number of mobile terminals are grouped into terminal groups and in which terminal-to-terminal communication is performed within each of the terminal groups is referred to as the “relay system based on the terminal group”.
In the “relay system based on the terminal group” described above, the End-To-End communication route from the transmission source terminal of the packet to the final destination terminal is not extended beyond the outer edge of the terminal group to which the transmission source terminal belongs. Specifically, the number of hops in the End-To-End communication route depends on the size of the terminal group to which the transmission source terminal belongs, and the size of the terminal group refers to the maximum value of the number of hops in the communication route from a child node terminal located on the outer edge of the terminal group to a GO which is a parent node.
In the “relay system based on the terminal group”, it is possible to appropriately select the size of the terminal group according to a wireless channel state and a radio wave condition within the adjacent area around the transmission source terminal and the presence density and the condition of the geographical arrangement of other mobile terminals. Hence, the size of the terminal group is appropriately selected according to the conditions as described above, and thus it is possible to appropriately restrict the upper limit of the number of hops in the End-To-End communication route according to the conditions. Consequently, it is possible to avoid the above-described problems caused by increasing the number of hops in the End-To-End communication route.
Hence, in the wireless network environment in recent years, as a packet relay system which can solve the problems in the scalability of the communication capacity, the efficiency of the communication, the optimization of the efficiency of the consumption of terminal power and the stability of the End-To-End communication route that are found in the “table drive-type” and “storage transfer-type” systems, the “relay system based on the terminal group” is regarded as promising.